24,376 research outputs found
Exact formula for currents in strongly pumped diffusive systems
We analyze a generic model of mesoscopic machines driven by the nonadiabatic
variation of external parameters. We derive a formula for the probability
current; as a consequence we obtain a no-pumping theorem for cyclic processes
satisfying detailed balance and demonstrate that the rectification of current
requires broken spatial symmetry.Comment: 10 pages, accepted for publication in the Journal of Statistical
Physic
Efficiency of molecular machines with continuous phase space
We consider a molecular machine described as a Brownian particle diffusing in
a tilted periodic potential. We evaluate the absorbed and released power of the
machine as a function of the applied molecular and chemical forces, by using
the fact that the times for completing a cycle in the forward and the backward
direction have the same distribution, and that the ratio of the corresponding
splitting probabilities can be simply expressed as a function of the applied
force. We explicitly evaluate the efficiency at maximum power for a simple
sawtooth potential. We also obtain the efficiency at maximum power for a broad
class of 2-D models of a Brownian machine and find that loosely coupled
machines operate with a smaller efficiency at maximum power than their strongly
coupled counterparts.Comment: To appear in EP
Quantum Equivalence and Quantum Signatures in Heat Engines
Quantum heat engines (QHE) are thermal machines where the working substance
is quantum. In the extreme case the working medium can be a single particle or
a few level quantum system. The study of QHE has shown a remarkable similarity
with the standard thermodynamical models, thus raising the issue what is
quantum in quantum thermodynamics. Our main result is thermodynamical
equivalence of all engine type in the quantum regime of small action. They have
the same power, the same heat, the same efficiency, and they even have the same
relaxation rates and relaxation modes. Furthermore, it is shown that QHE have
quantum-thermodynamic signature, i.e thermodynamic measurements can confirm the
presence of quantum coherence in the device. The coherent work extraction
mechanism enables power outputs that greatly exceed the power of stochastic
(dephased) engines.Comment: v2 contains style and figures improvements. Subsection III.D was
adde
The Origins of Computational Mechanics: A Brief Intellectual History and Several Clarifications
The principle goal of computational mechanics is to define pattern and
structure so that the organization of complex systems can be detected and
quantified. Computational mechanics developed from efforts in the 1970s and
early 1980s to identify strange attractors as the mechanism driving weak fluid
turbulence via the method of reconstructing attractor geometry from measurement
time series and in the mid-1980s to estimate equations of motion directly from
complex time series. In providing a mathematical and operational definition of
structure it addressed weaknesses of these early approaches to discovering
patterns in natural systems.
Since then, computational mechanics has led to a range of results from
theoretical physics and nonlinear mathematics to diverse applications---from
closed-form analysis of Markov and non-Markov stochastic processes that are
ergodic or nonergodic and their measures of information and intrinsic
computation to complex materials and deterministic chaos and intelligence in
Maxwellian demons to quantum compression of classical processes and the
evolution of computation and language.
This brief review clarifies several misunderstandings and addresses concerns
recently raised regarding early works in the field (1980s). We show that
misguided evaluations of the contributions of computational mechanics are
groundless and stem from a lack of familiarity with its basic goals and from a
failure to consider its historical context. For all practical purposes, its
modern methods and results largely supersede the early works. This not only
renders recent criticism moot and shows the solid ground on which computational
mechanics stands but, most importantly, shows the significant progress achieved
over three decades and points to the many intriguing and outstanding challenges
in understanding the computational nature of complex dynamic systems.Comment: 11 pages, 123 citations;
http://csc.ucdavis.edu/~cmg/compmech/pubs/cmr.ht
Learning probability distributions generated by finite-state machines
We review methods for inference of probability distributions generated by probabilistic automata and related models for sequence generation. We focus on methods that can be proved to learn in the inference
in the limit and PAC formal models. The methods we review are state merging and state splitting methods for probabilistic deterministic automata and the recently developed spectral method for nondeterministic probabilistic automata. In both cases, we derive them from a high-level algorithm described in terms of the Hankel matrix of the distribution to be learned, given as an oracle, and then describe how to adapt that algorithm to account for the error introduced by a finite sample.Peer ReviewedPostprint (author's final draft
- …